Cyclic variations in diesel engines fueled with biofuel–diesel blends: A review of statistical and wavelet spectrum analysis methods

Cyclic variations in compression ignition engines are undesirable because they reduce efficiency, increase emissions, and limit power output. In this context, the adoption of alternative fuels has gained significant attention as a potential strategy to improve combustion characteristics. Biofuels, including biodiesel and alcohol‐diesel blends derived from both edible and non‐edible feedstocks, serve as viable alternatives to fossil fuels due to their inherent oxygen content and enhanced combustion characteristics. This review systematically analyzes peer‐reviewed experimental studies from the past two decades to assess cyclic combustion variability in diesel engines powered by biofuel‐diesel blends, focusing on in‐cylinder pressure dynamics, indicated mean effective pressure (IMEP), pressure rise rate, and heat release characteristics, and comparing statistical and wavelet‐spectrum‐based diagnostic methodologies. Conventional statistical metrics, including the coefficient of variation (COV) and correlation‐based measures, primarily assess the overall magnitude of fluctuations but offer limited insight into the temporal evolution and predominant modes of cyclic combustion variability, which can be more effectively analyzed using advanced time‐frequency methods. Data indicate that optimal blends can decrease COV IMEP by 20–35% with particular injection procedures; however, statistical convergence (about 150 cycles) fails to disclose underlying oscillatory patterns. Conversely, Continuous Wavelet Transform (CWT)‐based methodologies, specifically Wavelet Power Spectrum (WPS) and Global Wavelet Spectrum (GWS), provide adaptive time‐frequency decomposition, discerning intermittent short‐period (3–10 cycles) and long‐period (>150 cycles) instability bands. Wavelet analysis reveals substantial reductions in spectral power (e.g., an almost 80% decrease under optimal injection pressure settings), validating enhanced combustion stability beyond the insights provided by COV alone.